Manufacturers of cellular telephones need to evaluate the quality of the loudspeaker and/or the ear speaker assembled inside a telephone. One way to test if the speaker is good is to carry out an evaluation of its acoustic response or the equivalent electrical impedance curve.
An example quality analysis test consists of looking at the amplitude variation of the resonance peaks. In fact, the frequency and amplitude of resonance peaks depend on the quality of speaker and are not disturbed by variations in speaker components. For calculation practical reasons, a method of looking at anti-resonance values may be used instead.
A circuit used to measure the impedance curve is illustrated in FIG. 1.
A resistor R1 is connected in series to the measured speaker 5 and an external voltage generator 10 supplies, or forces, a voltage Vin. The resistance of R1 may e.g. be selected as twice the DC resistance of the speaker.
Using a frequency sweep and measuring the voltage drop V1 across the resistor R1, it is possible to calculate the impedance Zspeaker versus frequency f using the following formula:
                                          Z            speaker                    ⁡                      (            f            )                          =                              R            1                    ·                      (                                                            V                  in                                                                      V                    1                                    ⁡                                      (                    f                    )                                                              -              1                        )                                              (                  Eq          .                                          ⁢          1                )            